javascriptのletとvarとconstキーワードの違いを知る

//    
'use strict';

(function() {
 var varTest;
 let letTest;
 console.log(varTest); //  undefined
 console.log(letTest); //  undefined
}());

//    
(function() {
 console.log(varTest); //  undefined(              )
 console.log(letTest); //    ：ReferenceError: letTest is not defined

 var varTest = 'test var OK.';
 let letTest = 'test let OK.';
}());

//    
'use strict';
(function() {
 var varTest = 'test var OK.';
 let letTest = 'test let OK.';
var varTest = 'varTest changed.';
 let letTest = 'letTest changed.'; //    ：SyntaxError: Identifier 'letTest' has already been declared
 console.log(varTest); //  varTest changed.(        letTest           )
 console.log(letTest);
}());

//    
'use strict';
(function() {
 var varTest = 'test var OK.';
 let letTest = 'test let OK.';
 {
 var varTest = 'varTest changed.';
 let letTest = 'letTest changed.';
 }
 console.log(varTest); //  "varTest changed."，  "{}"    varTest       letTest  
 console.log(letTest); //  "test let OK."，  "{}"    letTest    letTest       
}());

//    
const b = 2;//  
// const b;//  ，      
console.log('   const  b：' + b);//    
// b = 5;
// console.log('     const  b：' + b);//     

var a = 1;
// var a;//    
console.log('   var  a：' + a);//    a=1
function change(){
a = 4;
console.log('   var  a：' + a);//    a=4
} 
change();
console.log('     var  a        ：' + a);//    a=4

let c = 3;
console.log('   let  c：' + c);//  c=3
function change(){
let c = 6;
console.log('   let  c：' + c);//  c=6
} 
change();
console.log('     let  c          ：' + c);//  c=3